As a power source of a mobile device, or the like, a lithium ion secondary battery is mainly used. The function of the mobile device or the like is diversified, resulting in increasing in power consumption thereof. Therefore, a lithium ion secondary battery is required to have an increased battery capacity and, simultaneously, to have an enhanced charge/discharge cycle characteristic. Further, there is an increasing demand for a secondary battery with a high output and a large capacity for electric tools such as an electric drill and a hybrid automobile. In this field, conventionally, a lead secondary battery, a nickel-cadmium secondary battery, and a nickel-hydrogen secondary battery are mainly used. A small and light lithium ion secondary battery with high energy density is highly expected, and there is a demand for a lithium ion secondary battery excellent in large current load characteristics.
In particular, in applications for automobiles, such as battery electric vehicles (BEV) and hybrid electric vehicles (HEV), a long-term cycle characteristic over 10 years and a large current load characteristic for driving a high-power motor are mainly required, and a high volume energy density is also required for extending a driving range, which are severe as compared to mobile applications.
In the lithium ion secondary battery, generally, a lithium salt, such as lithium cobaltate, is used as a positive electrode active material, and a carboneous material, such as graphite, is used as a negative electrode active material.
Graphite is classified into natural graphite and artificial graphite. Among those, natural graphite is available at a low cost. However, as natural graphite has a scale shape, if natural graphite is formed into a paste together with a binder and applied to a collector, natural graphite is aligned in one direction. When an electrode made of such a material is charged, the electrode expands only in one direction, which degrades the performance of the electrode. Natural graphite, which has been granulated and formed into a spherical shape, is proposed, however, the resulting spherical natural graphite is aligned because of being crushed by pressing in the course of electrode production. Further, the surface of the natural graphite is active, and hence a large amount of gas is generated during initial charging, which decreases an initial efficiency and degrades a cycle characteristic. In order to solve those problems, Japanese Patent publication No. 3534391 (U.S. Pat. No. 6,632,569, Patent Document 1), etc. propose a method involving coating artificial carbon on the surface of the natural graphite processed into a spherical shape. The material produced by this method can address a high-capacity, a low-current, and an intermediate-cycle characteristic required by the mobile applications, etc. However, it is very difficult for the material to satisfy the requests such as a large current and an ultralong-term cycle characteristic of a large battery as described above.
On the other hand, regarding artificial graphite, first, there is exemplified a mesocarbon microsphere-graphitized article described in JP 04-190555 A (Patent Document 2). This is a well-balanced negative electrode material, and is capable of producing a battery with a high capacity and a large current. However, it is difficult to achieve the cycle characteristic for a much longer period of time than the one for mobile applications, which are required for a large battery.
Artificial graphite typified by graphitized articles, such as oil, coal pitch, and coke, is available at a relatively low cost. However, a satisfactory crystalline needle-shaped coke tends to align in a scale shape. In order to solve this problem, the method described in Japanese patent publication No. 3361510 (Patent Document 3) and the like yield results. This method can allow the use of not only fine powder of an artificial graphite material but also fine powder of a natural graphite, or the like, and exhibits very excellent performance for a negative electrode material for the mobile applications. This material can address the high-capacity, the low-current, and the intermediate cycle characteristic required for the mobile applications, etc. However, this material has not satisfied requests such as a large current and an ultralong-term cycle characteristic of a large battery as described above.
Further, negative electrode materials using so-called hard carbon and amorphous carbon described in JP 07-320740 A (U.S. Pat. No. 5,587,255, Patent Document 4) are excellent in a characteristic with respect to a large current and also have a relatively satisfactory cycle characteristic. However, the volume energy density of each of such negative electrode materials is too low and the price thereof is very expensive, and thus, such negative electrode materials are only used for some special large batteries.
On the other hand, though it has not attracted much attention to date, a carbon material obtained by the heat treatment at 1,600 to 2,300° C. of so-called easily-graphitizable carbon has a discharge capacity of about 250 mAh/g. Though it is much lower than a theoretical capacity of 372 mAh/g, the carbon material has an internal structure between a high cristallinity structure and a turbostratic structure: i.e. a structure on the way to a graphitized structure, and is known to have very excellent large current-input/output characteristics and cycle characteristics owing to the structure. However, it is almost impossible to improve a discharge capacity while maintaining large current-input/output characteristics. Also, when the material is mixed with natural graphite and the like having a high discharge capacity, it results in great decrease in the large current-input/output characteristics and a problem that it becomes impossible to keep the desired performance.